Adjustable resistor

ABSTRACT

An adjustable resistance device including a plurality of tubular casings formed of a material having a high rate of thermal conductivity. A resistance coil is wound around the outer surface of each casing and bonded to the casing by an electrically insulating, high-temperature binding material. A sliding contact is mounted for longitudinal movement along each resistance coil and by adjusting the position of the contact the portion of the resistance coil connected in the circuit can be varied. A fan or other fluid-moving device is associated with the tubular casings and acts to circulate air within the interior of the casings. Heat generated within the resistance coil is dissipated by conduction through the tubular casings and by convection to the air passing within the casings.

United States Patent [72] Inventor Lionel E. Weyenberg N35 W23575 Capitol Drive, Pewaukee, Wis. 53072 [2]] Appl. No. 778,731

[22] Filed [45] Patented Nov. 25, 1968 Sept. 28, 1971 [54] ADJUSTABLE RESISTOR 6 Claims, 2 Drawing Figs.

[52] US. Cl 338/53, 338/133, 338/226, 338/304 [51] Int. Cl H0lc 1/08 [50] Field ofSearch 338/53, 57, 58, 133,226, 270, 304

[56] References Cited UNITED STATES PATENTS 1,433,750 10/1922 Spooner 338/53 2,155,691 4/1939 Stevenson 338/53 X 2,668,218 2/1954 Searle 338/133 2,7l0,907 6/1955 Westbergetal. 3,248,680 4/1966 Ganci Primary ExaminerRodney D. Bennett, .lr. Assistant ExaminerRiehard E. Berger Attorney-Andrus, Sceales, Starke & Sawall ABSTRACT: An adjustable resistance device including a plurality of tubular casings formed of a material having a high rate of thermal conductivity. A resistance coil is wound around the outer surface of each casing and bonded to the casing by an electrically insulating, high-temperature binding material. A sliding contact is mounted for longitudinal movement along each resistance coil and by adjusting the position of the contact the portion of the resistance coil connected in the circuit can be varied.

A fan or other fluid-moving device is associated with the tubular casings and acts to circulate air within the interior of the casings. Heat generated within the resistance coil is dissipated by conduction through the tubular casings and by convection to the air passing within the casings.

ADJUSTABLE RESISTOR This invention relates to an adjustable resistance device and more particularly to an improved adjustable resistor having a high rate of heat dissipation.

ln electrical circuits, variable-resistance devices, such as power rheostats, may be connected in the circuit for varying the voltage, current or other characteristics of the circuit. A rheostat generally includes a resistance coil wound on an insulating core or support. The coil is connected in a circuit and a contact or wiper is slidably mounted on the coil and connected in the circuit to provide a means of adjusting the portion of the resistance coil which is connected in the circuit.

Rheostats are normally designed to carry substantial current and the current in the resistance wire generates heat is accordance with the square of the current and the resistance of the wire. As a result, substantial amounts of heat are generated within the coil and the amount of current or power that a rheostat can carry is directly dependent upon the dissipation of the heat generated.

The present invention is directed to an improved adjustable resistor having a high rate of heat dissipation thereby enabling the amount of current or power that the resistor can carry to be correspondingly increased. According to the invention the adjustable resistor includes a pair of tubular casings formed of a material having a high rate of thermal conductivity. The casings are disposed in spaced, side-by-side relation and a resistance coil is wound around the outer surface of each casing and bonded to the casing by an electrically insulated, high temperature, bonding composition. The coils are connected either in series or in parallel in an electrical circuit, and a sliding contact or brush is provided for each resistance coil. The brushes are carried by a brush holder which is mounted for longitudinal movement along the casings. By adjusting the position of the brushes, the portion of each resistance coil connected in the circuit can be varied.

A fan or other fluid moving device is associated with the tubular casing and acts to circulate air within the hollow interior of the casings. The heat generated within the resistance coils is rapidly distributed throughout the casings by conduction and is thendissipated to the air or other fluid moving within the casings by convection.

The resistor of the invention provides an improved rate of heat dissipation utilizing both conduction to the tubular casings as well as convention to the fluid passing within the easings. By substantially improving the rate of heat dissipation, the amount of current or power that the rheostat can carry can be correspondingly increased.

The resistor introduces a minimum amount of reaction into the circuit and can carry full wattage ratings with only small portions of the resistance coil connected in the circuit.

Other objects and advantages will appear in the course of the following description.

The drawings illustrate the best mode presently contemplated of carrying out the invention.

In the drawings:

FIG. 1 is a side elevational view of the rheostat of the invention; and

FIG. 2 is an end view with parts broken away in section.

The drawings illustrate an adjustable resistor or rheostat including a pair of tubular casings 1 which are disposed in parallel, side-by-side relation. The casings l are formed of a material having a high rate of thermal conductivity so that heat transferred to the casings can be rapidly dissipated. Metals such as aluminum, magnesium, copper and steel have been found to be particularly satisfactory for the casings 1. While metals are preferred due to their high rate of thermal conductivity and mechanical strength, other materials having a relatively high rate of thermal conductivity can also be utilized for the casings l.

The casings 1 are connected together by a pair of plates 2 which extend across the respective ends of the casings and are tied together by tie rods 3 which extend through the casings. Nuts 4 are engaged with the threaded ends of the tie rods 3.

A resistance coil 5 of helically wound metal wire disposed in closely spaced convolutions is bonded to the outer wall of each casing 1 by a layer of an electrically insulating cement or adhesive 6. The adhesive 6 can be any conventional adhesive having good electrically insulating properties as well as good heat transfer characteristics, in order to both insulate the coil 5 from the casings l and to rapidly transfer heat from the coil to the casing from which the heat is dissipated to the surrounding atmosphere. The adhesive 6 extends between the convolutions of the coil 5 to space the convolutions apart and physically lock the convolutions to the casing l.

A pair of terminal strips 7 and 8 are connected to the ends of each of the resistance coils 5 and the tenninal strips 7 and 8 are connected in an electrical circuit so that the coils 5 are arranged either in series or parallel.

The outer surface of each of the resistance coils 5 can be coated with an electrically insulating coating 9, and the coating 9 completely covers the coil 5 except for a strip of each coil 5 which is exposed and is engaged by a sliding contact or brush 10. The brushes 10 are mounted within a brush holder 11, and each brush 10 is biased outwardly toward the corresponding coil 5 by a coil spring 12 which is mounted in the recess in the brush holder and acts against the inner end of the brush.

The brush holder 11 is carried by a rod 13 which is mounted for sliding movement within a bushing 14 mounted in one of the connecting plates 2. To prevent the rod 13 from rotating with respect to the plate 2, the rod 13 is connected to the bushing by a key 15. As the rod 13 is moved with respect to the bushing 14 the brushes 10 will move along the exposed portions of the respective coils 5 to thereby vary the portion of each coil which is connected in the electrical circuit.

To aid in the dissipation of heat from the resistor, a fan 16 is mounted adjacent the open ends of the casings l and is driven by a motor 17. The fan acts to move air or other gas through the interior of the cylindrical casings l and thereby aids in dissipating heat from the casings by convection.

The current passing through the coils 5 generates heat in proportion to the square of the current and the resistance of the coil and thus, the rate of dissipation of the heat constitutes a limiting factor on the rating of the rheostat. In accordance with the invention, the heat generated within each coil 5 is transferred by the adhesive 6 to the casing l and then rapidly dissipated to the air moving within the casing. Thus the rheostat of the invention acts to dissipate heat, not only by conduction to the casings l but also by convection from the casings to the air passing through the interior of the casings. This results in an improved rate of heat dissipation from the resistor and therefore improves the overall wattage rating of the resistor over comparable prior art devices.

It has been found that using a series of wound casings 1, each of which has a relatively short axial length, provides substantially better heat dissipation than utilizing a single wound casing having an axial length equal to the sum of the length of the casings of the series.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. In an adjustable electrical resistance device, a plurality of tubular metallic casings having open ends and having a high rate of thermal conductivity, said casings disposed in spaced side-by-side relation, a resistance coil disposed in a helical pattern on the outer surface of each casing and connected in an electrical circuit, an electrically insulating bonding material securing each coil to the respective casing, each coil being out of direct contact with the respective casing and being insulated therefrom, a brush for each coil, brush-holding means for carrying said brushes, mounting means for mounting the brush-holding means for longitudinal movement with respect to said casings, movement of said brushes along said coils acting to vary the portions of the coils connected in said electrical comprises a rod mounted for sliding movement in said connecting means.

5. The device of claim 4, and including means for preventing rotation of said rod with respect to said connecting means.

6. The device of claim 1, and including means for urging each brush into contact with the respective coil. 

2. The device of claim 1, wherein said fluid-moving means is a fan.
 3. The device of claim 1, and including connecting means for connecting the corresponding ends of the casings together.
 4. The device of claim 3, wherein said mounting means comprises a rod mounted for sliding movement in said connecting means.
 5. The device of claim 4, and including means for preventing rotation of said rod with respect to said connecting means.
 6. The device of claim 1, and including means for urging each brush into contact with the respective coil. 